Pox diseases, Biology

Pox diseases

Small-pox in human beings and pox in a few animal species are closely related to each other. It is shown by the fact that the vaccine for the prevention of small pox consists of the virus of cow-pox collected from calves artificially infected with the disease. However, small pox has since been globally eradicated. Its other forms of animal pox viruses, viz. swine pox, horse pox, fowl pox and sheep pox with the possible exception of buffalo pox and campe pox are not transmissible to human beings. Pox diseases affecting human beings and animals have in all probability their origin in a virus analogous to cow pox.

Pox is transmitted by cutaneous inoculation or by smearing the infected material on an abraded surface of skin or on mucous membrane. In sheep, the infection is believed to be transmitted by inhalation since it is the lungs that are mostly affected. Pox viruses in a dry state can maintain their infectivity and virulence for several years at 4oC. In a wet state, the viruses are killed when exposed to 60oC for 10 min. The most effective and inexpensive chemical for inactivating pox viruses is potassium permanganate in a dilution of 1:1000. The majority of pox viruses can be grown on the chorio-allantoic membrane of fertile hen eggs. The lesions produced on the membrane are characteristic of a particular pox virus. Pox diseases can be easily reproduced in monkeys and rabbits.

The period of incubation in pox disease is 2 to 5 days. In cows, buffaloes, sheep and goats the disease usually commences with a rise in body temperature accompanied by marked constitutional disturbance. Five well marked stages of lesions viz. roseola stage consisting of diffused redness due to congestion and resembles a flea bite, which later turns into a raised hard area, the so-called popular stage. From the congested blood vessels in the papules, lymph exudes and vesicles are formed. The disease is now regarded as having reached the vesicular stage. In the next or the popular stage, the serous fluid becomes turbid with pus-producing organisms and the lesions are light yellow in appearance. The pustules dry up and form scabs in the last or the crust stage. The successive stages mentioned above appear at intervals of about 3 days and the full course run by the disease is of about 2 to 3 weeks. Recovered animals are usually resistant to further attacks of the disease.